In recent years, attention is attracted to storage systems directed to small-size, high-energy density uses, such as information-related devices and communication devices, that is, personal computers, video cameras, digital still cameras, cellular phones, etc., storage systems directed to large-size, power uses, such as electric vehicles, hybrid vehicles, fuel cell vehicle's auxiliary power supplies, power storage, etc. As one candidate therefor, nonaqueous electrolyte cells, such as lithium ion cells, lithium cells, lithium ion capacitors, etc., have actively been developed.
Of these nonaqueous electrolyte cells, there are many already put into practical use, but they are not satisfactory in various uses with respect to durability. Deterioration is severe, particularly when the environmental temperature is 45° C. or higher. Therefore, it is problematic in the use for being used in a high-temperature place for a long term, such as automotive use.
In general, in these nonaqueous electrolyte cells, a nonaqueous electrolyte or a nonaqueous electrolyte coagulated by a gelation agent is used as an ionic conductor. Its structure is as follows. As a solvent, there is used an aprotic solvent, for example, one type or a mixed solvent of several types selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, etc. As a solute, there is used a lithium salt, that is, LiPF6, LiBF4, (CF3SO2)2NLi, (C2F5SO2)2NLi, etc.
Hitherto, there have been studied the optimizations of various cell components, including active materials of cathode and anode, as means for improving durability, such as nonaqueous electrolyte cell's cycle characteristics, high-temperature storage stability, etc. Nonaqueous electrolyte-related technology is not an exception, either. There are proposals to make various additives suppress deterioration caused by the decomposition of an electrolyte on the surface of an active cathode or anode. For example, Patent Publication 1 proposes improving cell characteristics by adding vinylene carbonate to an electrolyte. This method prevents an electrolyte from decomposing on the surface of an electrode by coating the electrode with a polymer film by polymerization of vinylene carbonate. It is, however, also difficult for lithium ions to pass through this film, thereby increasing internal resistance. It is a task that input and output characteristics are disadvantageous.
Non-patent Publications 1-3 and Patent Publications 2 and 3 describe that, when adding boron and phosphorus complex salts having oxalic acid group to electrolytes, high-temperature cycle characteristics and output characteristics improve by the effects of films that are formed on electrode interfaces. However, their effects are not yet sufficient. Furthermore, when increasing the amounts of these complex salts having oxalic acid group, there is a risk that gas is generated by decomposition reactions other than the film-forming reaction to cause swelling and performance deterioration of batteries. When decreasing the amount of addition to prevent this, it becomes impossible to obtain the effect. It is a task.
Furthermore, Patent Publication 4 describes the use of an imide salt having a sulfonic acid ester group as an electrolyte, but does not describe the improvement of characteristics by the use as an additive.
Furthermore, Patent Publication 5 describes that, when using as an electrolyte an imide salt having a sulfonic acid ester group containing fluorine atoms, high voltage stability improves. The effect is, however, not yet sufficient, and there is no description about cycle characteristics.
Patent Publication 6 describes that a phosphoryl imide salt shows a good anticorrosive property as an electrolyte against aluminum as a cathode collector, but there is no proved examples. Furthermore, there is no description about the gas generation suppression effect by the addition.